The Pakistan Taliban has claimed responsibility for a string of attacks, singling out secular, left-leaning parties, which have been forced to scale back campaigning.

In a statement justifying the bombing campaign, which referenced the moral philosophers Kant and Bentham, the group blamed politicians for launching operations in the north-western tribal areas of the country.

“They are responsible for displacement of millions of people, for the plight of Pashtun nation, for their abandoned villages, for the destroyed mosques, for military operations and for the blood game since years,” it said.

“Now they are suffering for their sins, Tehreek-e-Taliban Pakistan will always avenge for the oppressed, insh’Allah.”

In the latest attack, six people died when a bomb exploded in Peshawar. Witnesses said the target appeared to be a police vehicle but a passing bus took most of the impact.

As a result the Awami National Party (ANP), which has seen its candidates targeted in the north-west, says it has cut back on election rallies and is focusing instead on door-to-door canvassing.

Similar tactics have been adopted by the Muttahida Qaumi Movement in Karachi.

Three bombs in the city killed at least three people on Saturday while a car bomb on Friday at an ANP meeting killed 10.

The Pakistan People’s Party has also been singled out and its leader, Bilawal Bhutto Zardari, the son of Benazir Bhutto, has been forced to issue speeches on video rather than attend rallies.

Ali Dayan Hasan, Pakistan director at Human Rights Watch, called on Pakistan’s interim government to provide security to high-profile candidates at risk of attack ahead of elections in May 11.

“Since the end of military rule in 2008, Pakistan’s political parties have displayed an impressive commitment to cementing democratic and constitutional rule,” he said.

“It would be a tragedy if a combination of militancy and the government’s failure to ensure security compromise the election and set back Pakistan’s progress towards regular, free, and fair elections in which all Pakistanis can participate.”